In order to prevent fuel being stolen from a vehicle key-operated caps have been proposed. A cap of this type basically comprises a gripping portion, facing the outside of the filler of the tank, and a locking or clamping portion, having first coupling means, which are adapted to co-operate with second coupling means provided at the filler of the tank. The coupling means referred to above are usually of a threaded or bayonet-coupling type.
The cap then comprises a locking device, which can be switched between a closing condition and an opening condition and comprises a rotor with a seat for a respective key. The device is operatively set between the gripping portion and the locking portion of the cap in such a way that, in the closing condition, a rotation movement imparted manually upon the gripping portion does not enable disengagement of the locking portion from the corresponding seat. Instead, in the opening condition of the mechanism, the aforesaid rotation movement enables disengagement of the clamping portion from its seat.
Key-operated caps are effective in the case of vehicles for private use, but are not completely suitable for preventing fraudulent removal of fuel from a vehicle to which a plurality of authorized persons have access. An example is the case of haulage companies, car-rental firms, and, more in general, all those organizations that have available vehicles that are normally used by different people. In these cases an authorized driver, or at least the person who has the keys of the vehicle at his disposal, can easily open the tank and fraudulently remove therefrom part of the contents. An example of fraud understood in this sense is the case in which the fuel that has been removed is replaced with another liquid, for example water. Problems of this sort are encountered in the case of tanks for additives, typically provided on certain types of vehicles such as lorries and buses.
Systems have been proposed for detecting opening of the cap of the tank of a vehicle, which are typically pre-arranged for detecting rotation of the cap and generating an alarm signal.
For example, GB-A-2343283 describes a remote detection system of this type. In this solution, the movement of the cap is detected by a sensor, and a transmitter circuit consequently sends an alarm signal to a control unit. In solutions of the type referred to above, the detection system can be incorporated in the cap. As mentioned previously, in key-operated caps the locking mechanism is designed in such a way that, when the mechanism itself is in the closing condition, the gripping portion is free to rotate even though this does not enable removal of the cap from the filler of the tank and hence removal of fuel: this fact may be the source of false alarms. For instance, if the vehicle is parked and somebody, for any reason, turns the cap, the system generates an alarm signal, even though in actual fact the cap has not been removed from the filler of the tank and no fraudulent action or removal of fuel is in progress.
In other known solutions the detection system comprises, instead, a sensor mounted at the filler. An arrangement of this type is described, for example, in WO03/047330, where associated to the cap is a magnet and associated to the filler of the tank is a magnetic sensor.
Solutions of this second known type can enable detection of an attempt at opening or the effective opening condition of the cap, but are not useful for ascertaining whether said opening is in actual fact followed by a fraudulent action or otherwise.